An ultrasonic motor disclosed in Japanese Patent Publication (Kokoku) No. H06-106028 uses a bar-like or plate-like vibrator having projections. An ultrasonic motor disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-111128 has a vibrator that has contact parts instead of projections.
The ultrasonic motor disclosed in Japanese Patent Publication (Kokoku) No. H06-106028 includes, as shown in FIG. 12, a vibration plate 51 which has one surface thereof formed with projections 52a and 52b, piezoelectric elements 53a, 53b, and 53c provided on the other surface of the vibration plate 51; and a driven member 55 disposed in contact with the projections 52a and 52b. The piezoelectric elements 53a, 53b, and 53c excite the vibration plate 51 to generate vibrations in a stretching vibration mode and a bending vibration mode. The projections 52a and 52bare disposed at the loops of vibrations in the stretching vibration mode or in the vicinity thereof so that the tip ends thereof make a reciprocating motion in a direction indicated by “X” in FIG. 12. The projections 52a and 52b are also disposed in the vicinity of the loops of vibrations in the bending vibration mode so that the tip ends thereof make a reciprocating motion in a direction indicated by “Z” in FIG. 12. Vibrations in the stretching vibration mode and vibrations in the bending vibration mode are combined to cause the tip ends of the projections 52aand 52b to make an elliptic motion. This elliptic motion causes a feeding force to be transmitted to the driven member 55 to move the driven member 55 relative to the vibration plate 51 in the direction indicated by “X” in FIG. 12.
The reason why the vibration plate 51 has the projections 52a and 52b that are disposed at loops of vibrations in the stretching vibration mode or in the vicinity thereof and are also disposed in the vicinity of loops of vibrations in the bending vibration mode is that contacting of the driven member 55 with the vibration plate 51 at a position other than these positions causes an inconvenience such as generation of abnormal noise or reduced output. An elliptic motion generated at a position other than these positions has magnitude and phase that are different from those of an elliptic motion that is generated at the loops of vibrations in the stretching vibration mode or in the vicinity thereof and in the vicinity of the loops of vibrations in the bending vibration mode. This even impedes movement of the driven member 55.
As shown in FIG. 13, the ultrasonic motor disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-111128 includes a frame-like base 60, a rectangular flat vibration plate 61 supported by the base 60 via four stays and having a substantially smooth surface without no projection or recessed portion, a piezoelectric element 63 provided on one surface of the vibration plate 61 at one end portion 62 thereof, and a driven member 65 disposed in contact with the other surface of the vibration plate 61.
The piezoelectric element 63 excites the vibration plate 61 to generate primary out-of-plane bending vibrations to cause the vibration plate 61 to be arcuately deformed. This deformation causes both ends of the vibration plate 61 to be displaced, thereby moving the driven member 65 relative to the vibration plate 61 in a direction indicated by the arrow in FIG. 13. This vibration plate 61 has no projection and thus is excellent in workability, thereby providing the advantage that the manufacturing process can be simplified and a required component accuracy can be easily obtained.
However, in the case of the ultrasonic motor disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-111128, the vibration plate 61 of the vibrator has only the out-of-plane bending vibration mode as a vibration mode in which it can be excited to generate vibrations, making it difficult to obtain a large force for feeding the driven member. Further, in the ultrasonic motor disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2001-111128, the contact parts at which the vibration plate 61 contacts the driven member 61 are edges of the vibration plate 61, which have a very small area. Thus, it is difficult to form the contact parts at which the vibration plate of the vibrator contacts the driven member so as to obtain a desired shape and a desired flatness with a high accuracy.
On the other hand, in the case of the ultrasonic motor disclosed in Japanese Patent Publication (Kokoku) No. H06-106028, the vibration plate 51 of the vibrator is excited in the two vibration modes, thus being free of the above-described problem, i.e. an insufficient output from an ultrasonic motor. Moreover, this ultrasonic motor also has the two projections 52a and 52b and thus does not suffer from the above-described problem of the shape and flatness of the contact parts. However, in the vibration plate 51, the projections 52aand 52b are generally prepared from a material by machining and this machining cuts off significant portions of the vibration plate 51 other than the projections. Thus, the vibration plate 51 tends to have a deformation due to machining. In addition, it is difficult to provide the machined portions with a required accuracy. Furthermore, the existence of the projections 52a and 52b increases the size of the vibrator, which is not suitable for designing the ultrasonic motor compact in size.